The strain gauge used for measuring strain and stress of a structure is configured such that a circuit pattern formed by a metal thin film of Cu—Ni series alloys or Ni—Cr series alloys is covered with a flexible polyimide or epoxy resin film. Such a strain gauge is employed by being bonded to an object with use of an adhesive. If the circuit pattern of the metal thin film is strained and deformed, change in resistance is generated and an amount of the strain can be measured thereby.
There is also a semiconductor strain gauge which uses, in place of the metal thin film, a semiconductor piezoresistive element formed by doping an impurity to a semiconductor such as silicon as a strain detection section. The resistance change rate for the strain of the semiconductor strain gauge is several 10 times as large as that of the strain gauge using the metal thin film and can measure a minute strain. Moreover, since the resistance change rate of the strain gauge of the metal thin film is small, an obtained electric signal must be amplified, requiring an amplifier. Since the resistance change of the semiconductor strain gauge is large, an obtained electric signal can be used without help of an amplifier. Alternatively, it is possible to prepare an amplifier circuit in a chip of the semiconductor strain gauge. Therefore, it is expected that use and usability of the strain sensor will be extended greatly. In the present specification, a “strain sensor” is used as a synonym to a “strain gauge.”
Impurity doping is performed on a silicon wafer using a semiconductor production technology. Then, a wiring is formed to produce a chip and to obtain a semiconductor strain gauge. It is important that the strain of the object to be measured is correctly transmitted to the chip (hereafter, called a “strain sensor chip”). The points here are modularization of the strain sensor chip and attachment to the object to be measured.
Patent Document 1 discloses a structure in which a semiconductor strain gauge is made into a practical module. FIG. 16A is a perspective view showing such the semiconductor strain gauge. The semiconductor strain gauge is formed on a surface of a silicon wafer. Then, the silicon wafer is etched until it comes to have a thickness of several micrometers to produce a chip. Thus, a strain sensor chip 52 is obtained. A wiring 53 is formed and is sandwiched by polyimide films 54 to obtain a semiconductor strain gauge 51. Since the strain sensor chip 52 and the wiring 53 are modularized, the semiconductor strain gauge can be treated like a conventional strain gauge.
Patent Document 2 discloses a strain detection sensor 56 in which a strain sensor chip 52 is bonded to a glass pedestal 57 with use of a low-melting-point glass 58. FIG. 16B shows a side view of the strain detection sensor 56. The glass pedestal 57 is fixed to an object to be measured by bolts etc. Resin adhesives are not inserted between the strain sensor chip 52 and the glass pedestal 57 and between the glass pedestal 57 and the object to be measured, respectively. Therefore, a temperature drift, which may be caused by a difference in thermal expansion coefficients between the adhesive resin and the strain detection sensor, can be avoided.    Patent Document 1: JP-A-2001-264188    Patent Document 2: JP-A-2001-272287